[BugFix] Emit valid Prometheus # TYPE lines for BE/CN JVM metrics (backport #75240)

[mergify]

Why I'm doing:

enable_jvm_metrics = true makes the BE/CN /metrics endpoint emit invalid
Prometheus text. Each JVM gauge is registered with its label baked into the
metric-name string, e.g.:

register_metric("jvm_heap_size_bytes{type=\"committed\"}", &gauge);

so the exposition prints that string verbatim after # TYPE :

# TYPE starrocks_be_jvm_heap_size_bytes{type="committed"} gauge   <-- illegal
starrocks_be_jvm_heap_size_bytes{type="committed"} 905969664

A # TYPE metric name must not contain a label set. Prometheus aborts the
entire scrape on the first text-format error, so flipping this flag on
takes the node's whole metric set offline (up == 0) — not just the JVM
series. promtool check metrics rejects it with invalid metric name in comment. Reproduced on 4.0.10/4.0.11 and main; the feature shipped in 4.0.0
(#62210).

What I'm doing:

Register each JVM gauge under a bare metric name plus a real type label via
MetricLabels, mirroring the existing stream_load_metrics pattern. The
metrics framework then emits one valid # TYPE <name> line followed by the
labelled samples:

# TYPE starrocks_be_jvm_heap_size_bytes gauge
starrocks_be_jvm_heap_size_bytes{type="committed"} 905969664
starrocks_be_jvm_heap_size_bytes{type="max"} 21474836480
starrocks_be_jvm_heap_size_bytes{type="used"} 256609448

The sample lines (name + labels) are byte-identical to before, so the
time-series identity is unchanged and existing dashboards/alerts keep working;
only the previously-malformed # TYPE comment is corrected. No metric is
renamed, so no docs change is required.

jvm_metrics_test is updated to look metrics up by name + label, plus a
regression guard asserting the old braced names no longer resolve (which would
mean the label was still baked into the name).

Fixes #75159

What type of PR is this:

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Does this PR entail a change in behavior?

• Yes, this PR will result in a change in behavior.
• No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• Interface/UI changes: syntax, type conversion, expression evaluation, display information
• Parameter changes: default values, similar parameters but with different default values
• Policy changes: use new policy to replace old one, functionality automatically enabled
• Feature removed
• Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr needs user documentation (for new or modified features or behaviors)
  • I have added documentation for my new feature or new function
  • This pr needs auto generate documentation
• This is a backport pr

Bugfix cherry-pick branch check:

• I have checked the version labels which the pr will be auto-backported to the target branch
  • 4.1
  • 4.0
  • 3.5

---

This is an automatic backport of pull request #75240 done by [Mergify](https://mergify.com).

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